Sir Keir Starmer has described the US president as being “on really good form”, rejecting claims that he is senile.
The prime minister, who met Joe Biden at a Nato summit in Washington DC, said his US counterpart had “shown incredible leadership”.
Mr Biden is coming under continuing pressure to step aside from the presidential race due to concerns over his mental fitness.
Earlier this week, the actor and significant party donor George Clooney became one of the most high-profile Democrats urging Mr Biden to quit the contest.
Writing in the New York Times, Mr Clooney said he loved the president and believed in his morals but that he could not win “the fight against time”.
Asked that an increasing number of people were implying that President Biden is senile, Sir Keir answered “no”, and praised the president’s understanding during their bilateral meeting.
Sir Keir said Mr Biden had been “across all the detail”, adding that they had “gone at pace through a number of the issues”.
“We covered a lot of ground – strategic ground, international stuff, not just Ukraine.
“He was on good form.”
Sir Keir said the visit was an opportunity to “be clear about the special relationship and about our long-standing commitment to Nato”.
Nato – the military alliance made up of 32 European and North American countries – expects its members to spend at least 2% of national income on defence.
The new Labour government has said it aims to increase the UK’s spending to 2.5%, however has not set a date for hitting the target.
The Conservatives have said failure to set out a timetable would “cast serious doubt” on the government’s commitment to security.
Pressed on the spending target, Sir Keir reiterated that his commitment to meeting the 2.5% was “iron-clad” but that he did not want to give “an arbitrary date” without setting out how it could be afforded.
The prime minister said his government would first do a strategic review, arguing it was important to look at the information that wasn’t available to him when he was in opposition.
“Simply plucking a date is the politics of yesterday.
“I am determined that we are seriously going to put the plan in place to reach that commitment.”
